It is known in the manufacturing of power distribution, apparatus to include, with power transformers, automatically controlled load tap changers that can adjust the voltage at which power is fed to factories, subdivisions, apartment houses, and other large loads, typically several times per day but as often as hundreds of times per day, in response to variations in the applied load. These variations in the applied load change the voltage drops across such substantially fixed resistances as distribution wiring; the changes in the voltage drops in turn demand compensating adjustments in transformer winding connections to minimize errors in the available voltage, with the intent of maintaining at each distributed load as close to a constant voltage as practicable.
Transformer winding switching is performed by devices known to the art as load tap changers, so called because they are engineered to switch from one tap to another on a transformer while carrying kiloamp-level current loads. The contact portion of a load tap changer (LTC) is in some embodiments fully immersed in one of several blends of mineral oil, where the term oil may refer to one of a variety of petroleum distillates which are in the liquid state at room temperature, for insulation, cooling, and reduction of arcing. Numerous petroleum distillates may be suited to particular applications, as determined by operating temperature range, viscosity requirements, water absorption, electrical properties such as dielectric coefficient, conductivity, and change in electrical properties with moisture concentration, temperature, and the like.
The non-oil-filled gas volume at the top of the open chamber in a tap changer, transformer, or other device is termed ullage. The pressure in the ullage in an LTC tends to change slowly with outside temperature, as the oil volume typically can provide a significant thermal reservoir.
Despite the presence of insulating oil, the immersed tap switching events can produce arcing, which tends to break down the oil, leaving contaminating particles as well as liquid and gas hydrocarbon molecules of various molecular weights. A portion of the contaminating particles can be deposited on the sliding contacts of the LTC, building up a resistive layer and increasing contact heating, with the waste heat ultimately coupled to the oil. Removal of these deposits is promoted by abrasion between the sliding contacts during each tap change. Another portion of the contaminating particles can remain in suspension in the oil until mechanically removed by passing the oil through a filter. Still another portion of the contaminating particles may sink to the bottom of the oil volume, while others float to the surface or form foams.
An LTC can be vented rather than being hermetically sealed, so that there is some opportunity in many systems for water vapor and other airborne contaminants to enter the system; the contaminants can be absorbed by the oil, can be entrained as corrosion promoters, and can be shown to directly lower the dielectric constant of the oil. A variety of known technologies can serve for suppression of entrainment of water vapor, such as the use of a desiccant within the ullage of the LTC.
Another phenomenon evident in some LTCs, in the presence of dissolved oxygen and water in mineral oil subjected to arcing events, is formation of organic acids and other reactive chemical compounds, some of which can be destructive of some components of the system.
Accordingly, there is a need in the art for an apparatus and method capable of providing to some extent a continuously refreshed nonreactive gas atmosphere in an LTC and associated subsystems, balancing requirements for fresh supplies of gas against assured minimization of combustibles, oxidizers, and other corrosives in all accessible regions of the LTC, both continuously during operation and at a rapidly restoring rate after servicing, while avoiding to at least some extent the requirement for periodic maintenance and its associated expenses.